Enzymes are used to process a variety of agricultural products such as wood, fruits and vegetables, starches, juices, and the like. Typically, processing enzymes are produced and recovered on an industrial scale from various sources, such as microbial fermentation (Bacillus α-amylase), or isolation from plants (coffee β-galactosidase or papain from plant parts). Enzyme preparations are used in different processing applications by mixing the enzyme and the substrate under the appropriate conditions of moisture, temperature, time, and mechanical mixing such that the enzymatic reaction is achieved in a commercially viable manner. The methods involve separate steps of enzyme production, manufacture of an enzyme preparation, mixing the enzyme and substrate, and subjecting the mixture to the appropriate conditions to facilitate the enzymatic reaction. A method that reduces or eliminates the time, energy, mixing, capital expenses, and/or enzyme production costs, or results in improved or novel products, would be useful and beneficial. One example of where such improvements are needed is in the area of corn milling.
Today corn is milled to obtain cornstarch and other corn-milling co-products such as corn gluten feed, corn gluten meal, and corn oil. The starch obtained from the process is often further processed into other products such as derivatized starches and sugars, or fermented to make a variety of products including alcohols or lactic acid. Processing of cornstarch often involves the use of enzymes, in particular, enzymes that hydrolyze and convert starch into fermentable sugars or fructose (α- and gluco-amylase, α-glucosidase, glucose isomerase, and the like). The process used commercially today is capital intensive as construction of very large mills is required to process corn on scales required for reasonable cost-effectiveness. In addition the process requires the separate manufacture of starch-hydrolyzing or modifying enzymes and then the machinery to mix the enzyme and substrate to produce the hydrolyzed starch products.
The process of starch recovery from corn grain is well known and involves a wet-milling process. Corn wet-milling includes the steps of steeping the corn kernel, grinding the corn kernel and separating the components of the kernel. The kernels are steeped in a steep tank with a countercurrent flow of water at about 120° F. and the kernels remain in the steep tank for 24 to 48 hours. This steepwater typically contains sulfur dioxide at a concentration of about 0.2% by weight. Sulfur dioxide is employed in the process to help reduce microbial growth and also to reduce disulfide bonds in endosperm proteins to facilitate more efficient starch-protein separation. Normally, about 0.59 gallons of steepwater is used per bushel of corn. The steepwater is considered waste and often contains undesirable levels of residual sulfur dioxide.
The steeped kernels are then dewatered and subjected to sets of attrition type mills. The first set of attrition type mills rupture the kernels releasing the germ from the rest of the kernel. A commercial attrition type mill suitable for the wet milling business is sold under the brand name Bauer. Centrifugation is used to separate the germ from the rest of the kernel. A typical commercial centrifugation separator is the Merco centrifugal separator. Attrition mills and centrifugal separators are large expensive items that use energy to operate.
In the next step of the process, the remaining kernel components including the starch, hull, fiber, and gluten are subjected to another set of attrition mills and passed through a set of wash screens to separate the fiber components from the starch and gluten (endosperm protein). The starch and gluten pass through the screens while the fiber does not. Centrifugation or a third grind followed by centrifugation is used to separate the starch from the endosperm protein. Centrifugation produces a starch slurry which is dewatered, then washed with fresh water and dried to about 12% moisture. The substantially pure starch is typically further processed by the use of enzymes.
The separation of starch from the other components of the grain is performed because removing the seed coat, embryo and endosperm proteins allows one to efficiently contact the starch with processing enzymes, and the resulting hydrolysis products are relatively free from contaminants from the other kernel components. Separation also ensures that other components of the grain are effectively recovered and can be subsequently sold as co-products to increase the revenues from the mill.
After the starch is recovered from the wet-milling process it typically undergoes the processing steps of gelatinization, liquefaction and dextrinization for maltodextrin production, and subsequent steps of saccharification, isomerization and refining for the production of glucose, maltose and fructose.
Gelatinization is employed in the hydrolysis of starch because currently available enzymes cannot rapidly hydrolyze crystalline starch. To make the starch available to the hydrolytic enzymes, the starch is typically made into a slurry with water (20–40% dry solids) and heated at the appropriate gelling temperature. For cornstarch this temperature is between 105–110° C. The gelatinized starch is typically very viscous and is therefore thinned in the next step called liquefaction. Liquefaction breaks some of the bonds between the glucose molecules of the starch and is accomplished enzymatically or through the use of acid. Heat-stable endo α-amylase enzymes are used in this step, and in the subsequent step of dextrinization. The extent of hydrolysis is controlled in the dextrinization step to yield hydrolysis products of the desired percentage of dextrose.
Further hydrolysis of the dextrin products from the liquefaction step is carried out by a number of different exo-amylases and debranching enzymes, depending on the products that are desired. And finally if fructose is desired then immobilized glucose isomerase enzyme is typically employed to convert glucose into fructose.
Dry-mill processes of making fermentable sugars (and then ethanol, for example) from cornstarch facilitate efficient contacting of exogenous enzymes with starch. These processes are less capital intensive than wet-milling but significant cost advantages are still desirable, as often the co-products derived from these processes are not as valuable as those derived from wet-milling. For example, in dry milling corn, the kernel is ground into a powder to facilitate efficient contact of starch by degrading enzymes. After enzyme hydrolysis of the corn flour the residual solids have some feed value as they contain proteins and some other components. Eckhoff recently described the potential for improvements and the relevant issues related to dry milling in a paper entitled “Fermentation and costs of fuel ethanol from corn with quick-germ process” (Appl. Biochem. Biotechnol., 94: 41 (2001)). The “quick germ” method allows for the separation of the oil-rich germ from the starch using a reduced steeping time.
One example where the regulation and/or level of endogenous processing enzymes in a plant can result in a desirable product is sweet corn. Typical sweet corn varieties are distinguished from field corn varieties by the fact that sweet corn is not capable of normal levels of starch biosynthesis. Genetic mutations in the genes encoding enzymes involved in starch biosynthesis are typically employed in sweet corn varieties to limit starch biosynthesis. Such mutations are in the genes encoding starch synthases and ADP-glucose pyrophosphorylases (such as the sugary and super-sweet mutations). Fructose, glucose and sucrose, which are the simple sugars necessary for producing the palatable sweetness that consumers of edible fresh corn desire, accumulate in the developing endosperm of such mutants. However, if the level of starch accumulation is too high, such as when the corn is left to mature for too long (late harvest) or the corn is stored for an excessive period before it is consumed, the product loses sweetness and takes on a starchy taste and mouthfeel. The harvest window for sweet corn is therefore quite narrow, and shelf-life is limited.
Another significant drawback to the farmer who plants sweet corn varieties is that the usefulness of these varieties is limited exclusively to edible food. If a farmer wanted to forego harvesting his sweet corn for use as edible food during seed development, the crop would be essentially a loss. The grain yield and quality of sweet corn is poor for two fundamental reasons. The first reason is that mutations in the starch biosynthesis pathway cripple the starch biosynthetic machinery and the grains do not fill out completely, causing the yield and quality to be compromised. Secondly, due to the high levels of sugars present in the grain and the inability to sequester these sugars as starch, the overall sink strength of the seed is reduced, which exacerbates the reduction of nutrient storage in the grain. The endosperms of sweet corn variety seeds are shrunken and collapsed, do not undergo proper desiccation, and are susceptible to diseases. The poor quality of the sweet corn grain has further agronomic implications; as poor seed viability, poor germination, seedling disease susceptibility, and poor early seedling vigor result from the combination of factors caused by inadequate starch accumulation. Thus, the poor quality issues of sweet corn impact the consumer, farmer/grower, distributor, and seed producer.
Thus, for dry-milling, there is a need for a method which improves the efficiency of the process and/or increases the value of the co-products. For wet-milling, there is a need for a method of processing starch that does not require the equipment necessary for prolonged steeping, grinding, milling, and/or separating the components of the kernel. For example, there is a need to modify or eliminate the steeping step in wet milling as this would reduce the amount of waste water requiring disposal, thereby saving energy and time, and increasing mill capacity (kernels would spend less time in steep tanks). There is also a need to eliminate or improve the process of separating the starch-containing endosperm from the embryo.